1. Field of the Invention
The present invention relates to an optical scanning imaging system, and more particularly, to an optical scanning imaging system capable of improving resolution.
2. Description of the Related Art
As multimedia-oriented society emerges rapidly, a large-sized and high quality display screen is required. Recently, emphasis is put on realization of natural color in addition to high resolution.
A light source such as a laser having high color purity should be used to realize perfect natural colors. One of apparatus realizing a screen using a laser is an optical scanning laser imaging system using a scanner. An optical scanning laser imaging system has mainly used a polygonal rotating mirror and a galvanometer mirror. The optical scanning laser imaging system using the polygonal rotating mirror or the galvanometer mirror is difficult to manufacture in a small size and a unit price thereof is high.
Considering above problems, the present applicant has suggested a laser imaging system using a scanner adopting a microelectromechanical system (MEMS), which is disclosed in a U.S. Pat. No. 6,636,339.
The laser imaging system using the MEMS scanner is one of promising projection display devices of small form factor, low power consumption, and natural color realization.
To realize a laser imaging system of a large-sized screen and high resolution by applying the MEMS scanner, a sufficient scanning speed, a wide scanning angle, and a large mirror size are required.
Since a laser beam is coherent light and diffraction occurs much while the laser beam propagates as the width of the laser beam is small, it is difficult to make the width of the laser beam infinitely small. Also, as is well known in the art, a light beam is not focused as a point due to diffraction nature thereof and thus there exists a limit of resolution. In addition, when the size of a light beam incident to a lens system is large, the size of the light beam focused gets small.
Therefore, it is required to make the size of the unfocused light beam large so as to raise resolution. A large scanning frequency and a large value θD are required to realize an image of high resolution. Image resolution of a laser beam scanning optical system is determined by an optical invariant represented by a product θD of the diameter D of a collimated beam and a beam scanning angle θ.
The performance of a raster scanning system is defined by θD [deg·mm]. A VGA class and a XGA class require θD of about 7.50 and about 12.0, respectively. It is known that a product θD of about 22.5 is required to realize a high definition class display. θ is a mechanical scanning angle (unit: degree) of a scanner in one direction and D is the width of a beam, i.e., the effective mirror size (unit: mm) of the scanner in a laser scanning apparatus.
To realize a high resolution imaging system, the mirror size of the MEMS scanner should be large. Also, to realize a large-sized, high resolution laser imaging system, a scanning speed should be fast.
However, when the mirror size is increased, it is difficult to make a maximum operation speed of the MEMS scanner fast due to a physical property such as the moment of inertia, so that a scanning speed gets slow. Therefore, it is difficult to make the mirror size large while making the scanning speed fast.
Also, it is impossible to make a scanning angle of the MEMS scanner infinitely large and thus the scanning angle is limited.
To realize a subminiature high performance laser beam projection display, the MEMS scanner capable of scanning in high speed is considered as a reliable beam scanning device, but it is not easy to design a high speed MEMS scanner realizing a high resolution image due to trade-off relationship between an operation speed, an operation angle, and a reflection mirror size in device design. Therefore, it is difficult to obtain an MEMS scanner capable of accomplishing a sufficient scanning speed and scanning angle to realize a high definition, high resolution display while meeting a sufficient mirror size using a current MEMS manufacturing technique.